Mine were plastic and I had to sand them down to fit a 1/4" thermowell. They were slightly bigger than 0.250"...
The aluminum started to anneal after about 100 batches. I was not planning on upgrading as I was going to replace my upper cross members with a double wide profile member (like a 2x4 instead of a 2x2) which were starting to sag. But I was the steel place picking up a section of stainless rectangular box to make new legs for my conical (cheaper to buy a 10 foot stick and make your own leg extensions than to buy the OEM extensions) and they offered me a screaming deal on 2x2 box section SS so I bought enough to build a new frame on the spot. It was too good a deal to pass up.
I'm glad I made the new frame as the SS is much nicer. It doesn't get nearly as hot and I was able to upgrade my solenoids as well as replace all my steel parts with SS ones. Overall, I'm much happier with the SS frame and do not recommend aluminum unless you have a bunch of it sitting around gathering dust. If you brew a lot, then aluminum isn't a great choice. If you brew about 10 batches a year, the aluminum is probably sufficient. If you brew 50 batches a year, then it will not hold up.
The aluminum frame was far lighter which was a bonus for lifting the rig onto the trailer for offsite brew days (think Big Brew etc), but it gets insanely hot on the top level and made it really easy to burn yourself. You do learn to not touch the frame but you still get the occasional rub then sizzle. Much less chance of this with SS. And I was always geeking out over the integrated gas beam so I was able to do that this time around.